A synonymous decomposition applies to the structure functions \(xg_1^p\)  and \(xg_1^d\) . To project onto the singlet
distribution \(\Sigma \) 
directly one usually needs
charged current structure functions, as measured in neutrino experiments [1]}
or at facilities
like the planned LHeC project [2]} or the EIC [3]} in the
unpolarized and polarized case, at high luminosity, with [4]}
\(\frac{1}{2}\left[W_2^{p,+} + W_2^{p,-}\right] &=& x \Sigma \) 
